Rust, acid, and heat resisting ferrous alloy containing chromium and tantalum



. ance property of the alloy when subjected to Patented- May 7, 1929.

UNITED STATES PATENT oFFicE.

PERCY A. E.- ARMSTRONG, OF- NEW YORK, N. Y.

RUST, ACID, AND nnarnnsrsrme rn'anous ALLOY- TANTALUM.

CONTAINING GHBOMIUM AND Application filedAugust 8, 1925.- Serial No.148,918.

My invention relates to ferrous alloys containing iron, chromium and tantalum, with high-temperatures. Phosphorus and sulphur should be kept low. v

My alloy is very resistant to stainlng by food acids and vinegar, such as are encountered in household or hotel service, it is; very resistant to nitric acid but not very reslstant to sulphuric acid, hydrochloric acid, or; hy-

drofluoric acid. v Tantalum has the desirable property of resisting mineral acids, probably due to passivity. Alloys of tantalum and iron do not seem to have similar properties Where the iron is in much the greater proportion.

Tantalum, chromium and iron alloys are very resistant to rust and weak acids, due I believe to a great readiness to become passlve. Tantalum imparts to chromium and iron great strength and toughness, particularly at high temperatures.

Alloysof chromium and iron containing about16% of chroinium'when annealed have a low proportional limit. The addition of 2% of tantalum to such alloys raises the proportional limit by almost 100%, thereby providing an alloy 1n the class of those having the strength of structural steel. 7 Very small quantities of tantalum have a noticeable efiect on chromium and iron alloys, and also upon chromium and steel. I- prefer to use tantalum up to about 10%; for most purposes tantalum up to 5% appears to be desirable. r v

Tantalum improves the hot working prop erties of the alloy, particularly when the tan- .talum is about 2%. When the tantalum content is carried up to. 10% the hot working properties of my allo are not markedly improved 'by the additional tantalum, and if with such high tantalum the chromium con 1 tent of the alloy is towards the? high limit, the

alloy seems to have too' glreat a quantity of alloying constituents wit the iron for very readyhot working. Thiscondition is more noticeable as the carbon is raised towards the .t ed condition, such as hardened, or hardened and drawn. Carbon has the effect of decreasing the rust resisting, acid resisting and heat resisting properties of the alloy, and should,

therefore, be kept low, unless the carbon isrequired for hardening or strengthening purposes.

With increase of the chromium content there is a corresponding increase in the quantity of carbon required toproduce substantial hardness on being quenched from high tem 1 peratures. V

If the silicon content is raised to 1% or 2% and lthe chromium content is about 14% to 16% and with the carbon low (under 330%) the alloy is apt to lack shock strength in the x.annealed condition, due in part to the high chromium content. When, however, 1% to 3% of tantalum is added to such an alloy there is a marked increase in shock resistance. The lower the carbon the morenoticeable this becomes. v

, My alloy contains, besides tantalum, chromium 7% to 25%, carbon .0 to 1.25% and with or without high melting point elements. Where added resistance 'to scaling at'high temperatures is required, silicon up to about 4.5% is used. Such high silicon, however, is not claimed herein, as the alloy comprising 1% or more of silicon will be covered in a separate application. In the present case, while silicon is not essential, same maybe made. use ofas a normal constituent of steel and up to about 1%.

The single figure of the accompanying. 9o drawing shows the approximate limits in accordance with my invention of carbon and lehromium content in ironwith an appreciiable addition oftantal um. Line A defines substantially. the approximate upper limit of carbon and chromium with minimum tan-, talum that can be used in iron to produce a. stain free and rust resisting alloy when hardened from high temperatures, often w higher than is necessary to produce hardness.

I preferto harden by quenching in oil from about 1900 F. Thistemperature is over the carbon transformation point, and insures a rapid and thorough solution of the carbides.

There will always befree carbide in the hard- 4 ened alloy, but less when hardened from such high temperatures than from lower temperatures closer to or only slightly over the up er critical point.

t is best practicenot to approach too closei 1y to the limit line A, but to keep the carbon and chromium somewhat under the maximum posslble limiting figures and with this in view I have defined by line- A the approximate upper limit of carbon and chromium with minimum tantalum, that I prefer to use for purposes where hardening or hardening and tempering are of importance. Within the field to the right of and below said line A no difficulty is experienced in obtaining the desired stain free, rust resisting and heat resistall purposes whenused in theannealed con-- ing qualities upon suitably heating and coolv ing the alloy.

tantalum that I desire to use when the alloy is to be used in the soft or annealed condition. The auxiliary or subordinate limits A. and B are. preferably made, use of in practice for melting specifications with possible variation up to lines A and B in the case of heat treated and annealed products respectively. For convenience of reference, I have designated the area AXYZ as (1; area AXYZ as at; area BXY as b; and area B XY as b.

I prefer to keep the carbon under 20% for dition, and below .10.% carbon if maximum rust and acid and heat resisting propertie are essential.

I restrict myupper limit of chromium to about 25% because beyond this quantity of chromium the rust, acid, and heat resisting properties are not sufiiciently enhanced to Justify the use of additional quantities of chromium.

The use of less than 7% of chromium requires additional elements to impart to the alloy rust, acid and heat resisting properties, that are of importance as far as this invention is concerned. I have, therefore, restricted my minimum chromium contentto about 7%.

When the carbon content is raised" above' the limit shown by line A on the drawing thealloy either is not sufiiciently acid, rust. and heat resisting for the purpose of this invention, or the alley becomes too hard for. ready workin Naturally t ese limits fast lines of demarcation, but are more in the nature of'zones.

My alloy in particularly useful in I the hardened and tempered condition for tools,

cutlery, balls, hardened or hardened and] valves and valve seats, rooftempered parts, 7 cast ngs, tubes,

ing cooking utensils, pans,

are not hard and wire, forgings, and all purposes that require machining or working in any way. I

The surface of the alloy should be sand blasted, pickeled, or cleaned if the alloy surface is desired to remain clean, and is to resist rust and corrosion by acid or water vapor, in the'form of steam, for example. The mill oxide need not be removed if the surface is to withstand hot oxidation.

I find that the high melting point metals, tungsten, molybdenum, columbium, vanadium,;zirconium, titanium, all have properties akin to tantalum in increasing the physical properties of my alloy in the soft or heat treated condition,'or when subjected to high'tempcratures. Under the latter condition nickel and cobalt are effective. Nickel should be kept very high or low, that is over 15% or under 5%. .With lownickel the scale.

resisting properties are adversely affected. I find that it detracts from the easy working of the alloy to. have more than 5% of the high melting point metals as additions, or for replacing in part the'necessary tantalum content, and it is preferable to keep such additions under 8%, particularlyif the chromium content is carried toward the upper end of my range.

For rustless iron or steel bars, my preferred composition is: I e

Carbon-under 20%.

Chromium 10% to 17%.

Tantalum 20% up tot 3%.

Silicon up to .50%.

Manganese up to .6()%.

Phosphorus and sulfur low.

Balance iron.

High melting point elements I prefer to be not greater than 3% total, or the total tantalum and high melting point elements not greater than about 3%.

For piercing into tubes, I'preferably make use of the above analysis, with the carbon below 10%, generally about .06%.

I claim:

1. A rust, heat and acid resisting ferrous alloy containing chromium about %25%, an appreciable quantity of carbon under 1.25%, and an appreciable quantity of tantalum under 10%.

2. A rust, heat and acid reslstlng ferrous alloy containing chromium about %25%,

ferrous an appreciable quantity of carbon under 1.25%, and about 2% of tantalum.

4. Rust, heat and acid resisting articles made from a ferrous alloy containing an appreciable quantity under 10% of tantalum,

and containing carbon and chromium within the herein identified area b. I

5. Rust, heat and and reslstlng articles made from a ferrous alloy containing an ap- I preciablc quantity under 10% of tantalum,

and containing carbon and chromium within the herein identified area b.

6. Rust, heat and acid resisting articles made from a ferrous alloy containing an ap preciable quantity-under 5% of tantalum,

and containing carbon and chromium within the herein identified area b.. Q

7. Rust, heat and acid resisting articles .made from a ferrous alloy containing an app1eciable quantity under 5% of tantalum, and containing carbon and chromium within the herein identified area Q.

ferrous alloy containing alloy containing chromium 10%-17%, tanta- 111m .2%-3,%, silicon up to .5%, manganese up to .670, an appreciable quantity of carbon under .1%,'and the principal part of the remainder iron. v

In testimony whereof, I have signed my name hereto. PERCY A E. ARMSTRONG. 

